The Viral Analysis Unit of the LMMN has evaluated over 3500 clinical samples including CSF, plasma and serum for the detection of JC Virus genome sequences. This finding is diagnostic for PML, a rare but mostly fatal disease of the brain, as well as indicating the viral course of infection in many tissue compartments. The LMMN receives many such samples from around the world in circumstances in which diagnosis and pathogenesis cannot be made on clear clinical findings alone. The VAU has made hundreds of determinations on such samples and made the diagnosis on complex cases that provided critical information to neurologists and their patients for future medical care and treatment. In a survey of hundreds of plasma samples, it became clear that over 2% of the population worldwide can be viremic and that approximately 1% of the population is persistently viremic. Such numbers in a well defined and reliable assay system. The nature of the treatment that such viremic patients may receive for underlying diseases particularly that are classified as autoimmune diseases i.e. Multiple Sclerosis, Rheumatoid arthritis, Crohn's disease, Systemic Lupus Erythymatosis. Of the 405 MS patients that developed PML while treated with natalizumab,over half were diagnosed with the data from the Viral Analysis Unit. There have been at least 16 of these patients that had been misdiagnosed and not given correct medical treatment until the VAU provided the correct evidence of PML. Also the cumulative longitudinal data from this work has established that many PML patients never clear their infection even if immune reconstitution is establihsed. In some cases, patients show clinical signs of disease years after the initial diagnosis. This occurs in both HIV/AIDS cases as well as MS cases of PML and warrants close neurological evaluation at annual or sooner time points. In addition to the clinical samples that are received in the laboratory, there is also assay development. During this year, there has been an advance made to the quantitative PCR assay for detection of viral DNA. This new multiplex assay not only detects viral DNA in clinical samples it also simultaneously identifies whether the viral DNA is a pathogenic variant for JCV and not the non-pathogenic variant. From one CSF for example, the assay will read out the copies of the viral genome and also distinguishes the genotypic variant without the need for time consuming and costly DNA sequencing. The Multiplex assay provides additional information for risk stratification of patients who are dependent on therapies for underlying diseases thus making therapies more safe. Using the validated qPCR and ELISA assays, we have recently published data in the New England J Medicine showing that a number of MS patients can be viremic in the plasma but test serologically negative. This indicates that serology does not always measure prior expsoure to JCV. We have futher recommended that risk stratification paramenters include direct testing for virus either through the qPCR Multiplex assay or other assays in order to insure that all patients who are at risk for PML fall into a category for more intense medical monitoring.